1. Field of the Invention
The present invention relates to a mold closure position detection method for a mold clamping apparatus, which method detects a mold closure position on the basis of a variation in a physical quantity because of closure of a mold.
2. Description of the Related Art
A conventional toggle-type mold clamping apparatus for clamping a mold of an injection molding machine is disclosed in, for example, Japanese Patent Publication (kokoku) No. 6(1994)-61806. As disclosed in this publication, a toggle-type mold clamping apparatus includes a toggle link mechanism which connects a movable platen for supporting a movable mold half and a crosshead advanced and retracted by a drive unit, and has a function of transmitting pressing force of the crosshead to the movable platen while amplifying the force. In such a mold clamping apparatus, when the toggle link mechanism is almost completely extended, a predetermined mold clamping force determined on the basis of an extension of tie bars is generated. As shown in FIG. 10, in a mold clamping operation, high-speed mold closing is typically performed from a mold open position Xa, and the operation mode is switched to low-speed, low-pressure mold closing at a predetermined low-speed, low-pressure changeover position Xb. The period during which the low-speed, low-pressure mold closing is performed serves as a mold protection zone, during which a molded product not having been properly ejected or the like is detected as a foreign object. When a predetermined high-pressure changeover position Xc is reached, the operation mode is switched to high-pressure mold clamping so as to clamp the mold under high pressure. In FIG. 10, Xd shows a mold clamping end position. Load torque T of a drive motor for driving the mold clamping apparatus changes as shown in FIG. 10 during the mold clamping operation.
Incidentally, unlike a direct-pressure-application-type mold clamping apparatus, because of its operation principle, a toggle-type mold clamping apparatus has a drawback in that slight expansion or contraction of a mold and tie bars, stemming from disturbing factors such as heating temperature of the mold and ambient temperature, causes a considerable variation in mold clamping force, which results in deterioration in quality, in particular at the time of molding of precision products. FIG. 11 shows a variation in mold clamping force Fm with time for the case where the correct value (target value) of mold clamping force Fm is 400 kN. As is apparent from FIG. 11, during a period in which the mold temperature elevates, the mold clamping force Fm increases from 400 kN to 500 kN because of thermal expansion of the mold. After completion of the temperature elevation, since heat is transferred from the mold to the tie bars, the tie bars expand, whereby the mold clamping force Fm gradually decreases. Notably, thermal expansion of the mold is a factor which increases the mold clamping force Fm, and thermal expansion of the tie bars is a factor which decreases the mold clamping force Fm.
As described above, in a toggle-type mold clamping apparatus, disturbing factors such as heating temperature of a mold and ambient temperature are influential factors which must be taken into consideration so as to accurately maintain the mold clamping force Fm. Japanese Patent Application Laid-Open (kokai) No. 62(1987)-32020 discloses a mold clamping force control method for a toggle-type mold clamping apparatus, which method can cope with such disturbing factors. In the disclosed method, the thickness of a mold or a mold clamping force during a molding operation is detected by means of mold-thickness detection means consisting of an optical or magnetic scale supported on a stationary mold plate and a position detector disposed on a movable mold plate, and a correction value determined from the detected thickness and its target value is fed back to mold-thickness adjustment means, whereby mold clamping force is maintained constant.
However, the conventional mold clamping force control method (mold closure position detection method) for a toggle-type mold clamping apparatus has the following problems.
First, as described above, in a toggle-type mold clamping apparatus, slight expansion or contraction of a mold results in a considerable variation in mold clamping force. Since the conventional mold clamping force control method detects the thickness of a mold (mold clamping force) by use of mold-thickness detection means consisting of a scale supported on a stationary mold plate and a position detector disposed on a movable mold plate; i.e., the method detects slight expansion and contraction, the method cannot accurately detect mold clamping force (mold closure position).
Second, since the thickness of a mold is detected directly, separate mold-thickness detection means such as a scale and a position detector are needed, leading to an increase in the number of parts, higher cost, and increased degree of complexity of configuration; in particular, an increased degree of complexity of the structure around a mold.